Optical prism for projecting image

ABSTRACT

An optical prism ( 1 ) for reflecting incident light from an incidence surface ( 2 ) suited for incidence of light from an LCD ( 20 ) or the like for at least twice before emitting the reflected light as emission light from a predetermined emission surface ( 4 ). The optical prism has side surfaces ( 6 L and  6 R) crossing the incidence and emission surfaces ( 2  and  4 ) and including receded surfaces  7 L and  7 R) formed such as not to impede an optical path of an effective size, the incident light being led along the optical. path from the incidence surface to the emission surface.

BACKGROUND OF THE INVENTION

The present invention relates to an optical prism which is used in anoptical system for projecting image, produced on a liquid crystal orlike image display, onto a viewer'eye ball retina. Such an optical prismis a main optical element in an image display unit in a head mounteddisplay (abbreviated HMD), in which the image display unit is held at apredetermined position by a frame member mounted on the viewer'head forsingle- or double-eye image viewing.

Recently, development of such HMDs is in progress, and these devices arefinding increasing applications in various fields, such as medical,construction and educational fields and also amusement field such asvarious game machines.

Meanwhile, there is a trend for developing VGA purpose devices which candisplay high quality images and also displays of data systems as aversion of OA (office automation) systems.

Many HMDs having been developed or proposed up to date, are ofdouble-eye viewing type having a left and right image display for theleft and right eyes for viewing these image displays with both eyes. ForOA purposes, on the other hand, HDMs of single-eye viewing type alsohave been proposed. Such an HMD permits a single eye to an imagedisplay, while permitting a keyboard and other operating sections to beoperated simultaneously using the other eye, forward and at-hand visualfield thereof being ensured to this end.

The OA and like purpose HMDs are desirably compact and handy inconstruction, so that they can be worn for use in a sense just likewearing glasses.

However, HMDS for general home applications are still in the stage ofdevelopment, and no proposal has been made for a product which is notonly handy, but also can sufficiently solve various specific technicalproblems posed in its manufacture.

Japanese Laid-Open Patent Publication No. 8-234147, for instance, showsan optical prism, which reflects reflected light from an incidencesurface, having been incident from a display face of an LCD or likedisplay, at least twice in itself before emitting the light from apredetermined emission surface to the outside such that the emittedlight is led to a viewer'pupil.

This optical prism is a one-piece member having a plurality ofnon-spherical reflecting surfaces, and satisfies various opticalrequirements even when constructed as a viewing system.

When constructing an HMD or the like by employing this optical prism,however, it is necessary to hold the optical prism and a liquid crystaldisplay in a regular positional relation with high accuracy in order toobtain an accurate projection liquid crystal display image onto theviewer's retina. It is an unsolved technical problem to ensure thepositioning accuracy and also facilitate the manufacture.

SUMMARY OF THE INVENTION

The present invention was made in view of the above circumstances, andits object is to provide an optical system of the type as described,which can ensure necessary accuracy of the positioning of it and animage display to each other and also facilitate manufacture when it isemployed as an optical element of OA and various other systems.

Features and advantages over the prior art of the present invention willbe described in the following:

(1) According to a first aspect of the present invention, there isprovided an optical prism for reflecting incident light from anincidence surface suited for incidence of light from a predetermineddisplay or like light source for at least twice before emitting thereflected light as emission light through a predetermined emissionsurface. The optical prism has side surfaces crossing the incidence andemission surfaces and including receded surfaces formed such as not toimpede an optical path of an effective size, the incident light beingled along the optical path from the incidence surface to the emissionsurface.

Heretofore, no optical prism of this type has been proposed, which has aspecific shape such as to permit size reduction of its entirety,including portions for positioning it and a display or the like relativeto each other and also maintaining the relative positions of it and thedisplay.

According to the present invention as in (1) above, the receded surfacesdo not impede the optical path of the effective size, along which lightfrom the display or like light source is led, and can be utilized forproviding portions for positioning the optical prism and the display orthe like to each other and maintaining the relative positions of thesecomponents. Besides, it is possible to provide portions which do notimpede size reduction of the overall structure.

(2) According to a second aspect of the present invention, there isprovided the optical prism according to (1), which is fabricated by aninjection molding process, and in which the side surfaces have a dieremoval slope of 1 to 10 degrees with respect to a die removal directionin the injection molding process.

Heretofore, no optical prism of this type has been proposed, which has aspecific shape suited for mass production.

According to the present invention as in (2) above, the optical prismhas a specific shape which, in addition to the effect obtainableaccording to the present invention as in (1) above, is suited for massproduction by an injection molding process.

(3) According to a third aspect of the present invention, the opticalprism according to (1) is provided having projections outwardlyprojecting from the receded surfaces at a predetermined positionthereof.

Heretofore, no optical prism of this type has been proposed, which has aspecific shape which permits size reduction of its entirety even in thecase where projections are provided as portions for positioning it andthe display or the like and also maintaining the relative positions ofthese components.

According to the present invention as in (3) above, the receded surfacesdo not impede the effective size optical path of light from the displayor like light source, and can be utilized for providing projectionsserving as portions for positioning the optical prism and the display orthe like relative to each other and also maintaining the relativepositions of these components. Besides, the size reduction is lessimpeded by the projections even through these projections outwardlyproject from the receded surfaces.

(4) According to a fourth aspect of the present invention, there isprovided the optical prism according to (3), which is fabricated by aninjection molding process, and the projections of which havepredetermined outer surface portions serving as eject pin contactsurfaces in the injection molding process.

Heretofore, no specific means in the optical prism has been proposedthat sets eject pin contact surfaces when fabricating the optical prismof this type in an injection molding process.

According to the present invention as in (4) above, in addition to theeffect obtainable according to the present invention as in (3) above,the eject pin contact surfaces concerning the pertinent injectionmolding process are set on predetermined portions of the projectionsurfaces. Thus, any push flaw by the eject pin in forced contact willnot deteriorate the performance of the optical prism utilizing theoptical properties thereof.

(5) According to a fifth aspect of the present invention, there isprovided the optical prism according to (3), wherein the projectionsinclude frame portions for holding the display or the like andpositioning portions for determining the position of the display or thelike relative to the optical prism.

Heretofore, no optical prism of this type has been proposed, in whichthe projections do not impede the size reduction of its entirety even ifthey are utilized as positioning portions for determining the positionsof a frame member supporting the display or the like and relative toeach other.

According to the present invention as in (5) above, the projectionswhich are provided by utilizing the receded surfaces, in addition to theeffect obtainable according to the present invention as in (3) above,less impede the size reduction even if they are utilized as positioningportions for determining the positions of a frame member supporting thedisplay or the like and the optical prism, because the extent of theirprojection can be controlled by the receded surfaces.

(6) According to a sixth aspect of the present invention, there isprovided the optical prism according to (5), wherein the projectionseach have one or more flat portions, some of the flat portions beingsubstantially parallel at a perpendicular to display mounting surfacesformed on the frame portions.

Heretofore, no optical prism of this type has been proposed, which has aspecific structure for ensuring necessary accuracy of positioning of itand a display relative to each other.

According to the present invention as in (6) above, in addition to theeffect obtainable according to the present invention as in (5) above, itis possible to readily ensure necessary accuracy of positioning of theoptical prism and the display relative to each other.

(7) According to a seventh aspect of the present invention, there isprovided the optical prism according to (5), wherein the projectionshave one or more flat surfaces, one of the flat surfaces being apositioning surface having a greater area than an opposite surfacebehind it, side surfaces defined between the positioning surface and theopposite surface being formed such that they have a predetermined dieremoval slope suited for injection molding in the fabrication of theoptical prism.

Heretofore, no specific structure of this type has been proposed whichpermits readily ensuring necessary accuracy of positioning of theoptical prism and the display relative to each other and also readilyfabricating the optical prism by injection molding.

According to the present invention as in (7) above, in addition to theeffect obtainable according to the present invention as in (5) above, itis possible to permit readily ensuring necessary accuracy of positioningof the optical prism and the display relative to each other and alsoreadily fabricating the optical prism by injection molding.

(8) According to an eighth aspect of the present invention, there isprovided the optical prism according to (5), wherein the positioningportions have protruding or receding portions corresponding to recedingor protruding portions of a frame member of the display or the like, theprotruding or receding portions of the positioning portions havingprotrusions or depressions for restricting the release of theirengagement with the receding or protruding portions of the frame memberat least in a particular direction.

Heretofore, no specific structure of this type has been proposed, whichpermits readily ensuring necessary accuracy of positioning of theoptical prism and the display relative to each other and also readilyassembling the two components.

According to the present invention as in (8) above, in addition to theeffect obtainable according to the present invention as in (5) above, itis possible to permit readily ensuring necessary accuracy of positioningof the optical prism and the display relative to each other and alsoreadily assembling the two components.

(9) According to a ninth aspect of the present invention, there isprovided the optical prism according to (8), wherein the protrusion ordepressions are formed as ant protrusions or ant notches correspondingto ant notches or ant protrusions as receding or protruding portions ofthe frame member.

Heretofore, no specific structure of this type has been proposed, whichpermits readily ensuring necessary accuracy of positioning of theoptical prism and the display relative to each other and also readilyassembling the two components.

According to the present invention as in (9) above, in addition to theeffect obtainable according to the present invention as in (8) above, itis possible to permit readily ensuring necessary accuracy of positioningof the optical prism and the display relative to each other and alsoreadily assembling the two components.

(10) According to a tenth aspect of the present invention, there isprovided the optical prism according to (8), wherein the positioningportions have elastic force receiving surfaces in contact with andreceiving elastic forces of elastic pieces, which are provided on theframe member such as to suppress displacement of the frame memberrelative to the positioning portions by elastic forces induced by theirown elastic deformation.

Heretofore, no specific structure of this type has been proposed, whichpermits readily ensuring necessary accuracy of positioning of theoptical prism and the display relative to each other and also readilyassembling the two components.

According to the present invention as in (8) above, in addition to theeffect obtainable according to the present invention as in (5) above, itis possible to permit readily ensuring necessary accuracy of positioningof the optical prism and the display relative to each other and alsoreadily assembling the two components.

(11) According to an eleventh aspect of the present invention, there isprovided an optical prism for reflecting incident light from anincidence surface suited for incidence of light from predetermineddisplay or like light source for at least twice before emitting thereflected light as emission light through a predetermined emissionsurface, wherein: the optical prism has side surfaces crossing theincidence and emission surfaces and including receded surfaces formedsuch as not to impede an optical path of an effective size, the incidentlight being led along the optical path from the incidence surface to theemission surface, and predetermined areas of the receded surfaces beingprovided with a surface roughening treatment or like surface treatment.

Heretofore, no specific arrangement for effectively evading ghostgeneration due to light side-wise entering the effective size opticalpath in the optical prism from the incidence surface to the emissionsurface has been proposed.

According to the present invention as in (11) above, light is preventedfrom side-wise entering the effective size optical path in the opticalprism from the incidence surface to the emission surface, thuspreventing the image viewing quality from being spoiled by ghosteffects.

(12) According to a twelfth aspect of the present invention, there isprovided the optical prism according to (11), which is fabricated by aninjection molding process, in which the surfaces provided with theroughening surface treatment or like surface treatment have a dieremoval slope of 3 to 20 degrees with respect to a die removal directionin a pertinent injection molding process.

Heretofore, no specific proposal of this type was made, which permitsimproving the processibility of fabrication of the optical prism by aninjection molding process.

According to the present invention as in (12) above, in addition to theeffect obtainable according to the present invention as in (11) above,it is possible to improve the processibility of fabrication of theoptical prism particularly in an injection molding process.

(13) According to a thirteenth aspect of the present invention, there isprovided an optical prism for reflecting incident light from anincidence surface suited for incidence of light from predetermineddisplay or like light source for at least twice before emitting thereflected light as emission light through a predetermined emissionsurface. The incidence and emission surfaces and also the reflectingsurfaces concerning the reflection are substantially quadrilateral asdefined by their edges.

Heretofore, no specific proposal of this type has been made, whichpermits ensuring necessary die processing accuracy when fabricating theoptical prism by an injection molding process.

According to the present invention as in (13) above, it is possible topermit readily ensuring necessary die processing accuracy whenfabricating the optical prism by an injection molding process and thusreadily ensuring necessary processing accuracy of the optical prismitself.

(14) According to a fourteenth aspect of the present invention, there isprovided an optical prism for reflecting incident light from anincidence surface suited for incidence of light from predetermineddisplay or like light source for at least twice before emitting thereflected light as emission light through a predetermined emissionsurface. The optical prism is fabricated by an injection moldingprocess, and a reference axis of either one of the incidence surface,emission surface and reflecting surfaces and a die removal direction ina pertinent injection molding process are substantially parallel to eachother.

Heretofore, no specific proposal of this type has been made, whichpermits improving the processibility of fabrication of the optical prismby an injection molding process.

According to the present invention as in (12) above, in addition to theeffect obtainable according to the present invention as in (11) above,it is possible to improve the processibility of fabrication of theoptical prism particularly in an injection molding process.

(15) According to a fifteenth aspect of the present invention, there isprovided an optical prism for reflecting incident light from anincidence surface suited for incidence of light from predetermineddisplay or like light source for at least twice before emitting thereflected light as emission light through a predetermined emissionsurface. The optical prism is fabricated by an injection moldingprocess, and a die split line in the injection molding process does notpass through the incidence and emission surfaces, the reflectingsurfaces and the edges of these surfaces except for the edge definingthe incidence and emission surfaces adjacent to each other and the edgedefining the reflecting surfaces adjacent to each other.

Heretofore, there has been no specific means for avoiding size increaseof the optical prism while ensuring the effective optical size thereofwhen fabricating the optical prism by an injection molding process.

According to the present invention as in (15) above, a die removal slopewhen fabricating the optical prism by an injection molding process isprovided on two optical surfaces with respect to a die split line, andit is thus possible to avoid size increase of the optical prism whileensuring the effective optical size.

(16) According to a sixteenth aspect of the present invention, there isprovided an optical prism for reflecting incident light from anincidence surface suited for incidence of light from a predetermineddisplay or like light source for at least twice before emitting thereflected light as emission light through a predetermined emissionsurface. The optical prism is fabricated by an injection moldingprocess, and a die split line concerning the injection molding processpasses through projections outwardly projecting form side surfacescrossing the incidence and emission surfaces and formed at adequatetransversal positions.

Heretofore, no specific structure of this type has been proposed, whichis suited for forming, when fabricating the optical prism by aninjection molding process, portions for positioning the optical prismand the display or the like relative to each other and also maintainingthe relative positions of these components.

According to the present invention as in (16), it is possible to readilyform, when fabricating the optical prism by an injection moldingprocess, the portions for positioning the optical prism and the displayor the like relative to each other and also maintaining the relativepositions of these components.

(17) According to a seventeenth aspect of the present invention, thereis provided the optical prism according to (16), wherein the projectionshave positioning regulation portions formed in their predeterminedportions and for regulating the positioning of a frame member supportingthe display or the like, the die slit line passing through predeterminedsurface portions of the positioning regulation portions.

Heretofore, no specific structure has been provided, which can ensure asufficient area as a predetermined area of a positioning regulationportion for regulating the positioning of a frame member supporting thedisplay or the like.

According to the present invention as in (17) above, in addition to theeffect obtainable according to the present invention as in (16) above,the predetermined area of the positioning regulation portion forregulating the positioning of the frame member supporting the display orthe like, is not restricted by the die removal slope necessary forfabricating the optical prism by an injection molding process, and it isthus possible to readily ensure a sufficient area.

(18) According to an eighteenth aspect of the present invention, thereis provided the optical prism according to (17), wherein predeterminedsurface portions of the projections other than the positioningregulation portions are set as eject pin contact surfaces concerning apertinent injection molding process.

Heretofore, no specific proposal of how to set an eject pin contactsurface concerning a pertinent injection molding process has been made.

According to the present invention as in (18) above, a predeterminedportion of the projection surface is set to be an eject pin contactsurface in the pertinent injection molding process, and even a push flawproduced by the eject pin in forced contact will not deteriorate theperformance of the optical prism utilizing optical properties thereof.

(19) According to a nineteenth aspect of the present invention, there isprovided the optical prism for reflecting incident light from anincidence surface suited for incidence of light from a predetermineddisplay or like light source for at least twice before emitting thereflected light as emission light through a predetermined emissionsurface. The incidence and emission surfaces and the reflecting surfacesas optical surfaces have flat surfaces of 1 millimeter or more indiameter formed outside effective size areas.

Heretofore, no specific proposal of the structure permitting measurementfor ensuring the accuracy of the shape and dimensions of the opticalprism has been made.

According to the present invention as in (19) above, the angulareccentricity of each optical surface of the optical prism can bemeasured by utilizing the flat surfaces of 1 millimeter or more formedoutside the effective size areas. It is thus possible to ensurenecessary accuracy of the shape and dimensions of the optical prismaccording to data of measurements.

(20) According to a twentieth aspect of the present invention, there isprovided an optical prism for reflecting incident light from anincidence surface suited for incidence of light from predetermineddisplay or like light source for at least twice before emitting thereflected light as emission light through a predetermined emissionsurface. Portions including predetermined portions having Imaginaryedges defined between adjacent imaginary surface portions of theincidence and emission surfaces adjacent to each other, and thereflecting surfaces adjacent to each other, except for effective sizeareas of these surfaces, are chamfered.

Heretofore, no specific structure of this type has been proposed, whichpermits minimizing the size and weight of the optical prism.

According to the present invention as in (20) above, it is possible toyet minimize the size and weight of the optical prism.

(21) According to a twenty first aspect of the present invention, thereis provided the optical prism according to (20), which is fabricated byan injection molding process, and in which a die split line concerningthe injection molding process passes through the chamfered portions.

Heretofore, no specific structure of this type has been proposed, whichpermits minimizing the size and weight of the optical prism.

According to the present invention as in (20) above, it is possible toyet minimize the size and weight of the optical prism.

(22) According to a twenty second aspect of the present invention, thereis provided an optical prism for reflecting incident light from anincidence surface suited for incidence of light from predetermineddisplay or like light source for at least twice before emitting thereflected light as emission light through a predetermined emissionsurface. Pertinent ones of the incidence and emission surfaces andreflecting surfaces as optical surfaces have indexes formed atpredetermined positions or optical on optical axis.

Heretofore, no specific proposal of the structure permitting measurementfor ensuring the accuracy of the shape and dimensions of the opticalprism has been made.

According to the present invention as in (22) above, the indexes can beutilized for measuring the position relation and angular eccentricity ofthe optical surfaces of the optical prism. It is thus possible to ensurethe accuracy of the shape and dimensions of the optical prism accordingto data of measurements.

Other objects and features will be clarified from the followingdescription with reference to attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are perspective views showing an embodiment of theoptical prism according to the present invention;

FIG. 2 is a view showing the optical path of an optical prism like thatshown in FIGS. 1A to 1C;

FIG. 3 is a schematic view showing an optical prism like that shown inFIGS. 1A to 1C, which is mounted together with a display and relatedcomponents in a case 40;

FIG. 4 is a view showing a viewer bearing a single-eye viewing headmounted device (HMD) comprising an image display unit 41, which includesan optical system for display, constructed by using the optical prismaccording to the present invention;

FIG. 5 is a schematic view showing the image display unit 41accommodating the optical system for display constructed by employingthe optical system 1 according to the present invention, the unit 41being used in the single-eye viewing HMD as described before inconnection with FIG. 4;

FIGS. 6A and 6B are views illustrating a manner of assembling an opticalprism 1 and an associated casing 30, like those described before inconnection with FIGS. 1A to 1C to 5;

FIGS. 7A to 7C are views illustrating a different example of the mannerof assembling an optical prism 1 and an associated casing 30, like thosedescribed before in connection with FIGS. 1A to 1C to 6A and 6B;

FIGS. 8A and 8B are views showing a further example of the manner ofassembling of an optical prism 1 and an associated casing 30, like thosedescribed before in connection with FIGS. 1A to 1C to 7;

FIGS. 9A and 9B are views showing a still further example of the mannerof assembling an optical prism and an associated casing, like thosedescribed before in connection with FIGS. 1A to 1C to 8A and 8B;

FIG. 10 is a side view for describing the shape and dimensions of theoptical prism shown in FIGS. 1A to 1C;

FIG. 11 is a front view for typically describing various features of theoptical prism 1 according to the present invention;

FIGS. 12A and 12B are views for describing the outer shape of theoptical prism according to the present invention and features of thepertinent injection molding process;

FIGS. 13A and 13B are views for describing local surface treatment anddie split line setting with an optical prism like that shown in FIGS.12A and 12B;

FIGS. 14A to 14C are views for describing the function of the localsurface treatment (roughening treatment) of the optical prism describedabove in connection with FIGS. 13A and 13B;

FIGS. 15A and 15B are views for describing a feature of the outer shapeof the optical prism according to the present invention;

FIGS. 16A to 16C show another embodiment of the present invention;

FIG. 17 is a view showing the optical path of an optical prism like thatshown in FIGS. 16A to 16C; and

FIGS. 18A and 18B are views showing a manner of assembling an opticalprism 100 and an associated casing 30, like those described before inconnection with FIGS. 16A to 16C.

PREFERRED EMBODIMENTS OF THE INVENTION

Preferred embodiments of the present invention will now be describedwith reference to the drawings.

FIGS. 1A to 1C are perspective views showing an embodiment of theoptical prism according to the present invention. The illustratedoptical prism is a three-reflection prism for reflecting a light beamfrom an incidence surface three times before emission.

FIG. 1A is a perspective view showing the optical prism with anincidence surface and an emission surface (which are rear surfaces)viewed obliquely downward from behind. FIG. 1B is a perspective viewshowing the optical prism with inner reflection surfaces (which arefront surfaces) viewed obliquely downward from ahead. FIG. 1C is a sideview showing the optical prism viewed in the width direction thereof.

Reference numeral 2 (see FIG. 1A) designates a rearwardly inclined oneof two surfaces defining the top edge of the optical prism 1. Thissurface 2 is an incidence surface (facing a display, such as an LCD,hereinafter referred to as D surface) which is formed such as to besuitable for incidence of light from the display. Reference numeral 3(see FIG. 1B) designates a forwardly inclined one of two surfacesdefining the top edge of the optical prism 1. This surface 3 is an innersurface serving as a reflecting surface (hereinafter referred to as Csurface), at which light beam from the D surface is reflected for thefirst time in the optical prism 1. Reference numeral 4 (see FIG. 1A) isa rear surface downwardly extending from a lower edge of the D surface2. This surface 4 is an emission surface (hereinafter referred to as Asurface). The inner side of the A surface 4 is a reflection surface, atwhich light beam having been reflected for the first time at the Csurface 3 is reflected for the second time in the optical prism 1.Reference numeral 5 designates a front surface extending downward from alower edge of the C surface 3. The inner side of this surface 5 is areflecting surface (hereinafter referred to as B surface), at which thelight beam having been reflected for the second time at the inner sideof the A surface 4 is reflected for the third time in the optical prism1. The D, C, A and B surfaces 2, 3, 4 and 5 are curved surfaces formedsuch as to satisfy necessary optical characteristics.

Left and right side surfaces 6L and 6R are formed such that they crossthe incidence and emission surfaces (i.e., D and A surfaces 2 and 4).Recessed surfaces 7L and 7R are formed on the sides of the two sidesurfaces 6L and 6R such that they do not interfere with an optical pathof an effective size, along which the incident light is led from theincidence surface (i.e., D surface 2) to the emission surface (i.e., Asurface 4). In the example shown in FIGS. 1A to 1C, a left and a rightstep surfaces 8L and 8R are formed such that they terminate in thereceded surfaces 7L and 7R. By these step surfaces 8L and 8R, an upperpart of the optical prism 1 with front and rear surfaces thereofconstituted by the C and D surfaces 3 and 2 is made narrower than alower part of the optical prism 1 with front and rear surfacesconstituted by the B and A surfaces 5 and 4.

Side projections 9L and 9R outwardly project from the receded surfaces7L and 7R such that they extend beyond the left and right side surfaces6L and 6R.

Three over-flow extensions 1 b downwardly extend from the lower part ofthe optical prism 1 with the front and rear surfaces constituted by theB and A surfaces 5 and 4. These over-flow extensions 1 b are formed whenfabricating the optical prism 1 by an injection molding process.

As has been described above in connection with FIGS. 1A to 1C, theincidence and emission surfaces (i.e., D and A surfaces 2 and 4) and thereflecting surfaces (i.e., B and C surfaces 5 and 3), are substantiallyquadrilateral as defined by their edges. Thus, it is possible to readilyensure necessary die processing accuracy when molding the optical prismby injection molding process and readily ensure necessary processingaccuracy of the optical prism itself.

FIG. 2 is a view showing the optical path of an optical prism like thatshown in FIGS. 1A to 1C. In FIG. 2, parts like those shown in FIGS. 1Ato 1C are designated by like reference numerals and symbols. A manner inwhich light beams are incident from a display face (or light source) 21of a display (for instance an LCD) and emitted to the pupil side aredepicted by three chain lines.

FIG. 3 is a schematic view showing an optical prism like that shown inFIGS. 1A to 1C, which is mounted together with a display and relatedcomponents in a case 40. In FIG. 3, parts like those shown in FIGS. 1Ato 1C are designated by like reference numerals and symbols. An LCD 20as a display having a display face 21 does display operation byreceiving illumination light from a backlight 22 disposed behind it.Behind the backlight 22 is disposed a circuit board 23, on which drivesystem circuits for the LCD 20 and the backlight 22 are mounted. The LCD20, the backlight 22 and the circuit board 23 are assembled in a casing30 as shown by dashed rectangle. As will be described later in detailthe casing 30 is positioned relative to and secured to the optical prism1 by the side projection 9L noted above.

FIG. 4 is a view showing a viewer bearing a single-eye viewing headmounted device (HMD) comprising an image display unit 41, which includesan optical system for display, constructed by using the optical prismaccording to the present invention.

With reference to the Figure, the HMD has a frame 10, which is assembledfrom a main frame 10M which is found at a position to be in contact withthe head 2F of the viewer M when the HMD is mounted and constitutes amain part thereof, and a left and a right rear frame 10L and 10R hingedby a left and a right hinge 10Lh and 10Rh, respectively, to the mainframe 10M.

Provided on an upper portion of the main frame 10M is a couplingmechanism 10C, which suspends the image display unit 41 at apredetermined position. The image display unit 41 displays on itsdisplay section images supplied to it via a cord 11 c as an image signaltransmission line. In this embodiment, the image display unit 41 has avisor 11V, which restricts the visible light permeability concerning theforward visual field of the eye not for image viewing with the imagedisplay unit to 70 per cent or below, so that image viewing may be madewithout being Interrupted. In other words, the visor 11V effectivelysuppresses visual field struggle between the two eyes at the time of theimage viewing.

The main frame 10M includes a head push member 10F, which is held inforced contact with front head portion 2F or a nearby portion of theviewer M with a predetermined force applied in a direction substantiallynormal to that portion, thus restricting relative movements of it andthat head portion of the viewer M in contact with each other.

As shown above, in this embodiment of the HMD the frame 10 is held at aprescribed position relative to the head of the viewer M by elasticforces of itself as a whole. The frame 10 thus uses no separate springor like elastic member and is simplified in construction, thus readilypermitting size and weight reductions.

In this embodiment, the above elastic forces are obtained by using aplastic material selected from the group consisting of polyamide,polycarbonate, polypropyrene, ABS, polyethylene, polyethylenetelephthalate and polyacetal for the left and right rear frames 10L and10R.

For the main frame 10M a carbon-containing engineering plastic materialis used.

By using the above materials for the frame 10, sufficient elastic forcesfor obtaining an adequate position restricted state can be obtained. Inaddition, by using the carbon-containing engineering plastic material,it is possible to obtain excellent rigidity and also reduce thepossibility of distortion of the frame during image viewing andhindering thereof.

The left and right rear frames 10L and 10R are coupled to the main frame10M by flexible coupling mechanisms. That is, as described before, inthis embodiment the left and right rear frames 10L and 10R are hinged bythe hinges 10Lh and 10Rh which serve as the coupling mechanisms to themain frame 10M.

FIG., 5 is a schematic view showing the image display unit 41accommodating the optical system for display constructed by employingthe optical system 1 according to the present invention, the unit 41being used in the single-eye viewing HDM as described before inconnection with FIG. 4. In FIG. 5, parts like those in FIGS 1A to 1C to4 are designated by like reference numerals and symbols, while omittingtheir detailed description.

In this example, the image display unit 41 has a case 40, which ismounted on the frame 10 (i.e., the coupling mechanism 10C thereof) toobtain the HMD as shown in FIG. 4.

FIGS. 6A and 6B are views illustrating a manner of assembling of anoptical prism 1 and an associated casing 30, like those described beforein connection with FIGS. 1A to 1C to 5.

FIG. 6A is a perspective view showing the optical prism 1 and the casing30 having been assembled together. FIG. 6B is an exploded perspectiveview showing the optical prism 1 and the casing 30 separately.

In FIGS. 6A and 6B, parts like those described before in connection withFIGS. 1A to 1C to 5 are designated by like reference numerals andsymbols, while omitting their detailed description.

As shown, the casing 30 is a hollow rectangular body having opticalsystem mounting members 31L and 31R outwardly projecting from theopposite sides. The optical system mounting members 31L and 31R havesubstantially central, through threaded holes 32L and 32R penetratingthem in the forward/rearward direction. The optical prism 1 hasprojections 9L and 9R provided on receded surfaces 7L and 7R. Theprojections 9L and 9R have through holes 9Lh and 9Rh (not shown)penetrating them in the forward/rearward direction. The through holes9Lh and 9Rh can be aligned to the through threaded holes 32L and 32R.The projections 9L and 9R of the optical prism 1 and the optical systemmounting members 31L and 31R of the casing 30 are coupled to one anotherby a screw 50L, which is passed through the through hole 9Lh and screwedthrough the through-threaded hole 32L, and another screw 50R (notshown), which is passed through the through hole 9Rh and screwed throughthe through-threaded hole 32R. In this way, the associated parts arepositioned relative to one another. The optical system mounting members31L and 31R have depressions corresponding to the projections 9L and 9Rof the optical prism 1. The projections 9L and 9R can be snugly fittedin the depressions, whereby the optical prism and the casing arepositioned relative to each other in the forward/rearward, transversaland vertical directions.

FIGS. 7A to 7C are views illustrating a different example of the mannerof assembling an optical prism 1 and an associated casing 30, like thosedescribed before in connection with FIGS. 1A to 1C to 6A and 6B.

FIG. 7A is a perspective view showing the optical prism 1 and the casing30 having been assembled together. FIG. 7B is an exploded perspectiveview showing the optical prism 1 and the casing 30 separately. FIG. 7Cis an enlarged-scale fragmentary sectional view taken in the directionof arrow A in FIG. 7B.

In FIGS. 7A to 7C, parts like those described before in connection withFIGS. 1A to 1C to 6A and 6B are designated by like reference numeralsand symbols, while omitting their detailed description.

As shown, the casing 30 is a hollow rectangular shape having opticalsystem mounting members 31La and 31Ra outwardly projecting from oppositesides. The optical system mounting members 31La and 31Ra have notches33L and 33R cut from the bottom and also protuberances 33La and 33Raprojecting into the notches 33L and 33R at a vertical intermediate levelposition thereof and serving as positioning click mechanisms.

The optical prism 1 has projections 9La and 9Ra provided on recededsurfaces 7L and 7R. The projections 9La and 9Ra are complementary inphase to the notches 33L and 33R, having depressions 9La1 and 9Ra1formed substantially of an intermediate level position corresponding tothe protubereances 33La and 33Ra as the positioning click mechanisms.When the projections 9La and 9Ra of the optical prism 1 are engaged inthe notches 33L and 33R of the optical system mounting members 31La and31Ra up to a regular position, the protuberances 33La and 33Ra as thepositioning click mechanisms are click engaged snugly and elastically inthe depressions 9La1 and 9Ra1 of the projections 9La and 9Ra of theoptical prism 1. In this way, the associated parts are reliablypositioned relative to one another in the forward/rearward, transversaland vertical directions. It is thus possible to obtain ready assemblingwithout use of any screw or other separate member. From FIG. 7C which isan enlarged-scale fragmentary sectional view taken in the direction ofarrows A in FIG. 7B, the manner of engagement between the projection 9Laof the optical prism 1, projecting upwardly or in the form of letter Lwhen viewed upwardly, and the associated notch 33L of the optical systemmounting member 31La.

It will be understood from the above description with reference to thedrawings and also illustrations thereof that, the projections 9L and 9R,or 9La and 9Ra, have one or more flat portions, some of which aresubstantially parallel or perpendicular to a display mounting surfaceformed in the casing 30 as a frame member. It is thus possible toreadily ensure necessary accuracy of positioning of the optical prismand the display relative to each other.

The positioning function portions of the optical prism 1 concerning theabove positioning of the associated parts, has protruding or recedingportions corresponding to receding or protruding portions formed on theside of the frame member or casing 30, and such protruding or recedingportions have protuberances or depressions for restricting the releasingof their engagement with the associated receding or protruding portionson the side of the frame member or casing 30 at least in a particulardirection. It is thus possible to readily ensure necessary accuracy ofpositioning of the optical prism and the display relative to each other,while permitting ready assembling of the optical prism and the display.

The above protuberances or depressions may be formed as antprotuberances or ant notches corresponding to ant notches or antprotuberances formed as the receding or protruding portions on the sideof the frame member or casing 30. Again with this arrangement it ispossible to readily ensure necessary accuracy of positioning of theoptical prism and the display relative to each other, while permittingready assembling of the optical prism and the display.

FIGS. 8A and 8B are views showing a further example of the manner ofassembling of an optical prism 1 and an associated casing 30, like thosedescribed before in connection with FIGS. 1A to 1C to 7.

FIG. 8A is a perspective view showing the optical prism 1 and the casing30 having been assembled together. FIG. 8B is an exploded perspectiveview showing the optical prism 12 and the casing 30 separately.

In FIGS. 8A and 8B, parts like those described before in connection withFIGS. 1A to 1C to 7, are designated by like reference numerals, whileomitting their detailed description.

As shown, the casing 30 is a hollow rectangular body having opticalsystem mounting member 31Lb and 31Rb projecting from the opposite sides.The optical system mounting members 31Lb and 31Rb have elastic portions34L and 34R each formed by cutting two parallel notched from the front.These elastic portions 34L and 34R have inner protuberances 34Lb and34Rb formed at their front or free end.

The optical prism 1 has projections 9La and 9Ra provided on recededsurfaces 7L and 7R. The projections 9L and 9R have front depressions 9Lband 9Rb formed at a vertically mid level position corresponding to theprotuberances 34Lb and 34Rb of the optical system mounting members 31Lband 31Rb. The protuberances 34Lb and 34Rb of the optical system mountingmembers 31Lb and 31Rb can be snugly and elastically click fitted in thedepressions 9Lb and 9Rb of the projections 9La and 9Ra of the opticalprism 1. It is thus possible to obtain reliable positioning in theforward/rearward, transversal and vertical directions and obtain readyassembling without use of any screw or like separate member.

FIGS. 9A and 9B are views showing a still further example of the mannerof assembling an optical prism and an associated casing, like thosedescribed before in connection with FIGS. 1A to 1C to 8A and 8B.

FIG. 9A is a perspective view showing the optical prism 1 and the casinghaving been assembled together. FIG. 9B is an exploded perspective viewshowing the optical prism 1 and the casing 30 separately.

In FIGS. 9A and 9B, parts like those described before in connection withFIGS. 1A to 1C to 8A and 8B are designated by like reference numerals,while omitting their detailed description.

As shown, the casing 30 is a hollow rectangular body having opticalsystem mounting members 31Lc and 31Rc projecting from the oppositesides. The optical system mounting members 31Lc and 31Rc haverectangular holes 35L and 35R formed near their front end. The opticalsystem mounting members 31 c and 31Rc are elastically coupled to thecasing 30, or they are elastic in themselves.

The optical prism 1 has projections 9Lc and 9Rc formed on recededsurfaces 7L and 7R. The projections 9Lc and 9Rc have a shape such thatthey can be snugly fitted in the rectangular holes 35L and 35R of theoptical system mounting members 31Lb and 31Rb. The projections 9Lc and9Rc can be snugly and elastically click fitted into the rectangularholes 35L and 35R of the optical system mounting members 31Lc and 31Rcof the casing 30. It is thus possible to obtain reliable positioning inthe forward/rearward, transversal and vertical directions and obtainready assembling without use of any screw or like separate member.

In the above examples shown in FIGS. 8A and 8B and also FIGS. 9A and 9B,the projections 9La and 9Ra or 9Lc an 9Lc serve as positioning members,and the elastic portions 34L and 34R as elastic members or the opticalsystem mounting members 31Lb and 31Rb provided on the frame member orthe casing 30 are engaged with the positioning members such as toprevent deviation from their positioned state relative to thepositioners by their own elastic forces. Thus, it is possible to obtainreliable positioning in the forward/rearward, transversal and verticaldirections and obtain ready assembling without use of any screw or likeseparate member.

FIG. 10 is a side view for describing the shape and dimensions of theoptical prism shown in FIGS. 1A to 1C. In FIG. 10, parts like thoseshown in FIGS. 1A to 1C are designated by like reference numerals andsymbols, while omitting their detailed description.

The D surface 2 which is formed such as to be suitable for incidence oflight from the LCD 20 as display, faces and is parallel to the displayface of the LCD 20. The surface of the projection 9R (and 9L as well) ofthe optical prism 1 that is in contact with the casing 30, serves as apositioning surface having a relatively large area. This surface facesand is parallel with the display face of the LCD 20 at a distance of0.05 millimeter therefrom. The positioning surface of the projection 9R(and 9L as well) has opposite edges PF which are spaced apart a distanceconforming to the corresponding dimension of the associated depressionon the side of the casing 30. The surface on the side of the projectionopposite the positioning surface has a relatively smaller area. That is,the distance Pd between the opposite edges of the opposite surface issmaller than the distance PD between the opposed edges PF of thepositioning surface.

As shown, the projection 9R (and 9L as well) has one or more flatsurfaces, one of which (i.e., the positioning surface between the edgesPF, PF) has a greater area than the opposite surface behind it. Withthis configuration, the side surfaces that are defined between thepositioning surface and the opposite surface therebehind have apredetermined die removal slope (i.e., a slope due to the differencebetween the distances PD and Pd) which is suited for the fabrication ofthe optical prism by injection molding.

With the structure as described above in connection with FIG. 10, it ispossible to permit readily ensuring necessary accuracy of positioning ofthe optical prism 1 and the display 20 relative to each other and alsoreadily fabricate the optical prism by injection molding.

FIG. 11 is a front view for typically describing various features of theoptical prism 1 according to the present invention.

Referring to the Figure, areas or zones shown enclosed in phantom linerectangles BLE1 and BLE2 on the C and B surfaces 3 and 5, the areasbeing receded a predetermined dimension from the edges of the C and Bsurfaces 3 and 5, are effective size areas of the C and B surfaces 3 and5. The phantom line rectangles BLE1 and BLE2 represent effective areaboundaries. It will be seen from FIG. 11 that with the optical prism 1according to the present invention, the recessed surfaces 7L and 7R areformed such that they do not impede the effective size optical path asshown enclosed within the phantom line rectangles BLE1 and BLE2.

In FIG. 11, the left side surface 6L has die removal slope GPL, which isprovided when fabricating the optical prism 1 by injection molding. Theright side surface 6R also has the same die removal slope. For obtainingsatisfactory processibility, the die removal slope GPL is set to anangle of 1 to 10 degrees with respect to the direction of die removal inthe pertinent injection molding process.

Of the surfaces of the projections 9L and 9R, which are formed such asto project from the receded surfaces 7L and 7R beyond the side surfaces6L and 6R, some surface portions other than those used positionregulating portions for regulating the position of the casing 30, areset as eject pin contact surfaces concerning the injection molding.

In this optical prism, flat surfaces PPL of 1 millimeter or more indiameter are further provided on the individual optical surfaces, i.e.,the incidence and emission surfaces (i.e., D and A surfaces 2 and 4) andalso reflecting surfaces (i.e., B and C surfaces 5 and 3), except forthe effective size optical path areas thereof.

Such flat surfaces PPL may be utilized for measuring the angulareccentricity of the individual optical surfaces of the optical prism,and it is possible to ensure necessary accuracy of the shape anddimensions of the optical prism according to data obtained by themeasurements.

In this optical prism, indexes (i.e., cross (+) position marks PMK inthe case of FIG. 11) are provided on the individual optical surfaces,i.e., the incidence and emission surfaces (i.e., D and A surfaces 2 and4) and reflecting surfaces (i.e., B and C surfaces 5 and 3), except forpredetermined position and optical axis (that is, except for theeffective size areas of the reflecting surfaces (i.e., B and C surfaces5 and 3) in the case of FIG. 11).

Such indexes (or position marks) PMK may be utilized for measuring thepositional relation and the angular eccentricity of the individualoptical surfaces of the optical prism, and it is possible to ensure theshape and dimensions of the optical prism according to data obtained bythe measurements.

FIGS. 12A and 12B are views for describing the outer shape of theoptical prism according to the present invention and features of thepertinent injection molding process.

FIG. 12A is a perspective view showing a manner of taking a sectionalview of the optical prism according to the present invention describedbefore in connection with FIGS. 1A to 1C. FIG. 12B is the sectional viewof the part shown in FIG. 12A. In FIGS. 12A and 12B, parts like thosedescribed before in connection with FIGS. 1A to 1C are designated bylike reference numerals and symbols, while omitting their detaileddescription.

The plane (i.e., H plane) along which to take the section shown in FIG.12A, is set to be parallel to the die moving direction (or die removaldirection) when fabricating the optical prism by the injection moldingprocess. Thus the direction of the broken line in the sectional view ofFIG. 12B is the die removal direction, which is perpendicular to a diesplit line (or plane) PL. As shown, the receded surfaces 7L and 7R havedie removal slopes K1 and K2 when fabricating the optical prism 1 by theinjection molding process. While only the die removal slopes K1 and K2of the left side receded surface 7L of the optical prism 1 are labeledas such, the right side receded surface 7R also has the same die removalslopes.

FIGS. 13A and 13B are views for describing local surface treatment anddie split line setting with an optical prism like that shown in FIGS.12A and 12B.

FIG. 13A is a front view showing an optical prism according to thepresent invention as described before in connection with FIGS. 1A to 1C.FIG. 1B is a side view showing a portion shown in FIG. 13A. In FIGS. 13Aand 13B, parts like those described before in connection with FIGS. 1Ato 1C, 12A and 12B are designated by like reference numerals andsymbols, while omitting their detailed description.

Referring to FIGS. 13A and 13B, shaded areas AVE have been subjected toa surface roughening treatment. The roughened surfaces have the dieremoval slopes K1 and K2 described before in connection with FIG. 12B.Satisfactory processibility is obtainable by setting the die removalslopes K1 and K2 to 3 to 20 degrees.

Referring to FIG. 13B, as in FIGS. 12A and 12B, a die split line (orplane) is labeled by PL.

It is a feature of the present invention that the die split line PL doesnot pass through the incidence and emission surfaces (i.e., D and Asurfaces 2 and 4) and reflecting surfaces (i.e., B and C surfaces 5 and3) as the optical surfaces and also the edges of these optical surfacesexcept for those defined between adjacent ones the optical surfaces.With this arrangement, the die removal slope when fabricating theoptical prism by an injection molding process, is provided from twooptical surfaces with respect to the die split line. It is thus possibleto evade size increase of the optical prism while ensuring the effectiveoptical size.

As shown in FIG. 13B, the die split line PL is set such that it passesthrough the projection 9R (and 9L as well). With this arrangement, it ispossible, when fabricating the optical prism by the injection moldingprocess, to readily form the projections which serve as portions forpositioning the optical prism and the display or the like reactive toeach other and also maintaining the relative positions of thesecomponents.

Particularly, the die split line PL is set such that it passes through apredetermined surface portion of the projection 9R (and 9L as well) thatserves the position regulation. This means that the predeterminedsurface portion of the position regulation portion for determining theframe member of the display or the like, can readily have a sufficientarea without being restricted by the die removal slope that is necessaryfor fabricating the optical prism by the injection molding process.

FIGS. 14A to 14C are views for describing the function of the localsurface treatment (roughening treatment) of the optical prism describedbefore in connection with FIGS. 13A and 13B.

FIG. 14A is a schematic side view depicting a manner, in which a lightbeam from point a of the display face of the display or the like isreflected at points b to d and then emitted in direction e. FIG. 14B isa schematic front view corresponding to FIG. 14A. In FIG. 14B, the lefthalf concerns a case of an optical prism without any die removal slopeprovided, and the right half concerns an optical prism with die removalslope provided. In the case where the die removal slope is provided,reflection at point d1, corresponding to reflection at point d in thecase where no die removal slope is provided, is not total reflection,but light partly leaks out in direction e1 (side-wise of the opticalprism), giving rise to ghost effects in image viewing. FIG. 14C is aschematic front view concerning the case where the roughening treatmentas described above in connection with FIGS. 13A and 13B is provided. Inthis case, in reflection at the roughened surface portion AVE, light ispartly emitted outward as shown at b2. The emitted light re-enters theoptical prism and leaks out in direction e2 (side-wise of the opticalprism) without reflection at point d1. Undesired light beam that causesghost effects thus can be greatly reduced.

FIGS. 15A and 15B are views for describing a feature of the outer shapeof the optical prism according to the present invention. The opticalprism shown in FIGS. 15A and 15B is like that shown in FIGS. 1A to 1C,and parts like those in FIGS. 1A to 1C are designated by like referencenumerals and symbols, while omitting their detailed description.

FIG. 15A is a perspective view showing the optical prism with theincidence and emission surfaces (i.e., rear surfaces) thereof viewedobliquely above from behind. FIG. 15B is a side view of the opticalprism.

As shown, predetermined portions (i.e., two portions) of imaginary edgeportions (as shown shaded) having imaginary surface portions of theincidence and emission surfaces (i.e., D and A surfaces 2 and 4) as theoptical surfaces, and those of the reflecting surfaces (i.e., B and Csurfaces 5 and 3), other than the effective size areas, the adjacentimaginary surface portions defining imaginary edges CUF (in this examplesubstantially triangular pyramidal portions as shown by broken lines),are chamfered. By adopting this shape, it is possible to further reducethe size and weight of the optical prism.

FIGS. 16A to 16C show another embodiment of the present invention. Inthe preceding embodiment of the optical prism, as described above inconnection with FIG. 2, the incident light is reflected three timesbefore being emitted. In this embodiment, the incident light isreflected twice before being emitted, as will be described hereinunder.

FIG. 16A is a perspective view showing this embodiment of the opticalprism with an emission surface (or rear surface) viewed from above andbehind. FIG. 16B is a perspective view showing the optical prism with anincidence surface and a reflecting surface (or rear surfaces) viewedfrom above and ahead. FIG. 16C is a side view showing the optical prismviewed in the width direction thereof.

Reference numeral 12 (see FIG. 16B) designates a forwardly inclined oneof two surfaces defining the top edge of the optical prism 100. Thissurface 12 is an incidence surface (facing a display, such as an LCD)which is formed such as to be suitable for incidence of light from thedisplay. Reference numeral 14 (see FIG. 16A) designates a rearwardlyinclined one of the two surfaces defining the top edge of the opticalprism 100. This surface 14 is a reflecting/emission surface serving bothas a reflecting surface, at which light beam incident from the surface12 facing the display is reflected for the first time in the opticalprism 100, and as an emission surface, through which light beam isemitted from the optical prism 100 to the outside.

Reference numeral 13 designates a surface extending downward from alower edge of the surface 12 facing the display. This surface 13 isspecifically an inner reflecting surface, at which a light beam havingbeen reflected at the inner side of the reflecting/emission surface 14is reflected for the second time in the optical prism 100. The surface12 facing the display, the reflecting surface 13 and thereflecting/emission surface 14 are curved surfaces formed such as tosatisfy necessary optical characteristics.

Left and right side surfaces 60L and 60R are formed such that they crossthe incidence surface 12 (facing the display), the reflecting/emissionsurface 14 and the reflecting surface 13. Recessed surfaces 70L and 70Rare formed on the side of the two side surfaces 60L and 60R such thatthey do not interfere with an optical path of an effective size, alongwhich the incident light is led from the incidence surface 13 to thereflecting/emission surface 14.

In the example shown in FIGS. 16A to 16C, a left and a right stepsurface 80L and 80R are formed such that they terminate in the recededsurfaces 70L and 70R. By these step surfaces 80L and 80R, an upper partof the optical prism 100 with front and rear surfaces thereofconstituted by the reflecting/emission surface 14 and the incidencesurface 12 is made narrower than a lower part of the optical prism 100with front and rear surfaces thereof constituted by thereflecting/emission surface 14 and the reflecting surface 13.

Projections 90L and 90R outwardly project from the receded surfaces 70Land 70R beyond the side surfaces 60L and 60R.

As has been described above in connection with FIGS. 16A to 16C, theincidence surface 12 (facing the display), the reflecting/emissionsurface 14 and the reflecting surface 13, are substantiallyquadrilateral as defined by their edges. Thus, it is possible to readilyensure necessary die processing accuracy when molding the optical prismby the injection molding process and readily ensure necessary processingaccuracy of the optical prism itself.

FIG. 17 is a view showing the optical path of an optical prism like thatshown in FIGS. 16A to 16C. In FIG. 17, parts like those shown in FIGS.16A to 16C are designated by like reference numerals and symbols. Amanner in which a light beam from a display face (or light source) of adisplay (for instance an LCD) is incident on the incidence surface 12(facing the display), reflected for the first time at the inner side ofthe reflecting/emission surface 14, then reflected from the second timeat the inner side of the reflecting surface 13 and then emitted throughthe reflecting/emission surface 14 to the pupil side, is depicted bythree chain lines. As is readily understood from FIG. 17, the opticalprism 100 is of twice reflection type.

FIGS. 18A and 18B are views showing a manner of assembling an opticalprism 100 and an associated casing 30, like those described before inconnection with FIGS. 16A to 16C.

FIG. 18A is a perspective view showing the optical prism 100 and thecasing 30 having been assembled together. FIG. 18B is an explodedperspective view showing the optical prism 100 and the casing 30separately.

In FIGS. 18A and 18B, parts like those described before in connectionwith FIGS. 18A and 18B are designated by like reference numerals andsymbols.

As shown, the casing 30 has a hollow rectangular body having opticalsystem mounting members 31L and 31R outwardly projecting from theopposite sides. The optical system mounting members 31L and 31R havesubstantially central, through threaded holes 32L and 32R penetratingthem in the forward/rearward directions. The optical prism 100 hasprojections 90L and 90R provided on receded surfaces 70L and 70R. Theprojections 90L and 90R have through holes 90Lh (not shown) and 90Rhpenetrating them in the forward/rearward direction. The through holes90Lh and 90Rh can be aligned to the through threaded holes 32L and 32R.The projections 90L and 90R of the optical prism 100 and the opticalsystem mounting members 31L and 31R of the casing 30 are coupled to oneanother by a screw 50L (not shown, which is passed though the throughhole 90Lh and screwed through the through threaded hole 32L, and a screw50R, which is passed through the through hole 90Rh and screwed throughthe through threaded hole 32R. The optical system mounting members 31Land 31R have depressions corresponding to the projections 90L and 90R ofthe optical prism 1. The projections 90L and 90R can be snugly fitted inthese depressions, whereby the optical prism and the casing arepositioned relative to each other in the forward/rearward, transversaland vertical directions.

As described above, the three-times reflection prism and the twicereflection prism have substantially the same features, although they aredifferent in their functional parts for obtaining their opticalcharacteristics (such as optical face). The above detailed descriptionof the three-times reflection prism is thus substantially applicable tothe twice reflection prism as well

As has been described in the foregoing, according to the presentinvention, when applied to optical elements of OA and various othersystems, it is possible to realize an optical prism, which can ensurenecessary accuracy of positioning of it and an image display relative toeach other in a pertinent system and be readily fabricated.

Changes in construction will occur to those skilled in the art andvarious apparently different modifications and embodiments may be madewithout departing from the scope of the present invention. The matterset forth in the foregoing description and accompanying drawings isoffered by way of illustration only. It is therefore intended that theforegoing description be regarded as illustrative rather than limiting.

What is claimed is:
 1. An optical prism comprising: an incidence surfaceconstructed and arranged to transmit light from a light source; anemission surface arranged with respect to said incidence surface to emitlight from said light source after having been transmitted through saidincidence surface and reflected at least twice within said opticalprism; and a side surface crossing said incidence and emission surfacesand including a receded surface formed to be non-impeding of an opticalpath of incident light traveling between said incidence surface to saidemission surface, wherein said optical prism is fabricated by aninjection molding process, and said side surfaces have a die removalslope of 1 to 10 degrees with respect to a die removal direction in theinjection molding process.
 2. An optical prism for a head mounteddisplay device comprising: an incidence surface constructed and arrangedto transmit light from a light source; an emission surface arranged withrespect to said incidence surface to emit light from said light sourceafter having been transmitted through said incidence surface andreflected at least twice within said optical prism; and a side surfacecrossing said incidence and emission surfaces and including a recededsurface formed to be non-impeding of an optical path of incident lighttraveling between said incidence surface to said emission surface,wherein said optical prism has projections outwardly projecting fromsaid receded surface at a predetermined position thereof.
 3. The opticalprism according to claim 2, which is fabricated by an injection moldingprocess, and the projections of which have predetermined outer surfaceportions serving as eject pin contact surfaces in the injection moldingprocess.
 4. The optical prism according to claim 2, wherein said lightsource is a display, and the projections include frame portionsconstructed and arranged to hold the display and positioning portionsfor determining the positions of the display relative to the opticalprism.
 5. The optical prism according to claim 4, wherein theprojections each have one or more flat portions, some of the flatportions being substantially parallel at a perpendicular to displaymounting surfaces formed on the frame portions.
 6. The optical prismaccording to claim 4, wherein the projections have one or more flatsurfaces, one of the flat surfaces being a positioning surface having agreater area than an opposite surface behind it, side surfaces definedbetween the positioning surface and the opposite surface being formedsuch that they have a predetermined die removal slope suited forinjection molding in the fabrication of the optical prism.
 7. Theoptical prism according to claim 4, wherein the positioning portionshave protruding or receding portions corresponding to receding orprotruding portions of a frame member of the display, the protruding orreceding portions of the positioning portions having protrusions ordepressions for restricting the release of their engagement with thereceding or protruding portions of the frame member at least in aparticular direction.
 8. The optical prism according to claim 7, whereinthe protrusions or depressions are formed as ant protrusions or antnotches corresponding to ant notches or ant protrusions as receding orprotruding portions of the frame member.
 9. The optical -prism accordingto claim 7, wherein the positioning portions have elastic forcereceiving surfaces in contact with and receiving elastic forces ofelastic pieces, which are provided on the frame member such as tosuppress displacement of the frame member relative to the positioningportions by elastic forces induced by their own elastic deformation. 10.An optical prism for reflecting incident light from an incidence surfacesuited for incidence of light from a light source for at least twicebefore emitting the reflected light as emission light through apredetermined emission surface, wherein: the optical prism has sidesurfaces crossing the incidence and emission surfaces and includingreceded surfaces formed to be non-impeding of an optical path of aneffective size, the incident light being led along the optical path fromthe incidence surface to the emission surface, and predetermined areasof the receded surfaces being provided with a surface rougheningtreatment.
 11. The optical prism according to claim 10, which isfabricated by an injection molding process, in which the surfacesprovided with the surface roughening treatment have a die removal slopeof 3 to 20 degrees with respect to a die removal direction in apertinent injection molding process.
 12. A head mounted display device,comprising: a head frame; and an image display unit attached to saidhead frame, wherein said image display unit comprises an optical systemhaving an optical prism according to claim
 10. 13. A head mounteddisplay device comprising: a head fame; and an image display unitattached to said head frame, wherein said image display unit comprisesan optical system having an optical prism, said optical prismcomprising: an incidence surface constructed and arranged to transmitlight from a light source; an emission surface arranged with respect tosaid incidence surface to emit light from said light source after havingbeen transmitted through said incidence surface and reflected at leasttwice within said optical prism; and a side surface crossing saidincidence and emission surfaces and including a receded surface formedto be non-impeding of an optical path of incident light travelingbetween said incidence surface to said emission surface, wherein saidoptical prism is fabricated by an injection molding process, and saidside surfaces have a die removal slope of 1 to 10 degrees with respectto a die removal direction in the injection molding process.
 14. A headmounted display device according to claim 13, wherein said image displayunit includes a casing having said light source disposed therein, saidcasing being attachable to said optical prism.